Questionable error messages from backend connections

Running tests using mysqlslap results in errors:
2015-07-07 04:26:21 Backend hangup error handling.
2015-07-07 04:26:21 Backend hangup -> closing session.
2015-07-07 04:26:27 Hangup in session that is not ready for routing, Error reported is ' [Pðÿ^?'.
2015-07-07 04:26:29 Backend hangup error handling.
2015-07-07 04:26:29 Backend hangup -> closing session.
etc.

The error the quotes peculiar characters is infrequent, but the others repeat many times.

The error messages are not clear as to what the error is, and it is unclear whether they are genuinely errors. Needs investigation.